Electrolytic process for the surface modification of high modulus carbon fibers

ABSTRACT

An improved electrolytic process is provided for modifying the surface characteristics of an electrically conductive high modulus graphitic carbonaceous fibrous material (i.e., exhibiting a mean single filament Young&#39;&#39;s modulus of at least about 60 million psi) and to thereby facilitate enhanced adhesion between the fibrous material and a resinous matrix material. The electrolytic process is carried out upon immersion of the fibrous material in an aqueous electrolytic solution of sodium hypochlorite (as defined) provided at a moderate temperature for a relatively brief residence time (i.e., about 2 to 10 minutes) while subjected to a relatively low current density (i.e., about 2.5 to 12 milliamps per square centimeter of surface area of fibrous material), and thereafter washing the same. Composite articles of enhanced interlaminar shear strength may be formed by incorporating the fibers modified in accordance with the present process in a resinous matrix material.

United States Patent 1191 Druin et al.

1 1 Jan.7, 1975 ELECTROLYTIC PROCESS FOR THE SURFACE MODIFICATION OFHIGH MODULUS CARBON FIBERS [75] lnventors: Melvin L. Druin, West Orange;

Andrew H. Diedwardo, Parisppany; James A. Parker, Somerville, all of NJ.

[73] Assignee: Celanese Corporation, New York,

22 Filed: Sept. 25, 1972 21 Appl. No.: 292,005

[52] US. Cl. 204/130 [51] Int. Cl C0ld 7/34, 801k 1/00 [58] Field ofSearch ..204/130, 132,211,206

[56] References Cited UNITED STATES PATENTS 1,543,357 6/1925 Baur204/129 1,793,914 2/1931 Dorsey 204/206 3,657,082 4/1972 Wells et al204/130 3,671,411 6/1972 Ray et a1 204/130 3,746,506 7/1973 Aitken etal. 204/130 3,759,805 9/1973 Chapman et al 204/130 PrimaryE.\aminer--J0hn H. Mack Assistant E.taminerR. L. Andrews [57] ABSTRACTAn improved electrolytic process is provided for modifying the surfacecharacteristics of an electrically con ductive high modulus graphiticcarbonaceous fibrous material (i.e., exhibiting a mean single filamentYoungs modulus of at least about 60 million psi) and to therebyfacilitate enhanced adhesion between the fibrous material and a resinousmatrix material. The electrolytic process is carried out upon immersionof the fibrous material in an aqueous electrolytic solution of sodiumhypochlorite (as defined) provided at a moderate temperature for arelatively brief residence time (i.e., about 2 to 10 minutes) whilesubjected to a relatively low current density (i.e., about 2.5 to 12milliamps per square centimeter of surface area of fibrous material),and thereafter washing the same. Composite articles of enhancedinterlaminar shear strength may be formed by incorporating the fibersmodified in accordance with the present process in a resinous matrixmaterial 16 Claims, 1 Drawing Figure SOURCE ELECTROLYTIC PROCESS FOR THESURFACE MODIFICATION OF HIGH MODULUS CARBON FIBERS BACKGROUND OF THEINVENTION In the search for high performance materials, considerableinterest has been focused upon carbon fibers. Graphite fibers orgraphitic carbonaceous fibers are defined herein as fibers which consistessentially of carbon and have a predominant x-ray diffraction patterncharacteristic of graphite. Amorphous carbon fibers, on the other hand,are defined as fibers in which the bulk of the fiber weight can beattributed to carbon and which exhibit an essentially amorphous x-raydiffraction pattern. Graphitic carbonaceous fibers generally have ahigher Youngs modulus than do amorphous carbon fibers and in additionare more highly electrically and thermally conductive.

Industrial high performance materials of the future are projected tomake substantial utilization of fiber reinforced composites, andgraphitic carbonaceous fibers theoretically have among the bestproperties of any fiber for use as high strength reinforcement. Amongthese desirable properties are corrosion and high temperatureresistance, low density, high tensile strength, and high modulus.Graphite is one of the very few known materials whose tensile strengthincreases with temperature. Uses for graphitic carbonaceous fiberreinforced composites include recreational equipment such as golf clubshafts, aerospace structural components, rocket motor casings, deepsu'bmergence vessels ablative materials for heat shields on re-entryvehicles, etc.

In the prior art numerous materials have been proposed for use aspossible matrices in which graphitic carbonaceous fibers may beincorporated to provide reinforcement and produce a composite article.The matrix material which is selected is commonly a thermosettingresinous material and is commonly selected because of its ability toalso withstand highly elevated temperatures.

While it has been possible in the past to provide graphitic carbonaceousfibers of highly desirable strength and modulus characteristics,difficulties have arisen when one attempts to gain the full advantage ofsuch properties in the resulting fiber reinforced composite article.Such inability to capitalize upon the superior single filamentproperties of the reinforcing fiber has been traced to inadequateadhesion between the fiber and the matrix in the resulting compositearticle.

Various techniques have been proposed in the past for modifying thefiber properties of a previously formed carbon fiber in order to makepossible improved adhesion when present in a composite article. See, forinstance, US. Pat. No. 3,476,703 to Nicholas J. Wadsworth and WilliamWatt wherein it is taught to heat a carbon fiber normally within therange of 350. to 850C. (e.g., 500 to 600C.) in a gaseous oxidizingatmosphere such as air for an appreciable period of time. Otheratmospheres contemplated for use in the process include an oxygen richatmosphere, pure oxygen. or an atmosphere containing an oxide ofnitrogen from which free oxygen becomes available such as nitrous oxideand nitrogen dioxide. Such hot gas techniques while effectivelyimproving bonding characteristics have been found, however, commonly tohave a tendency to concomitantly decrease the carbon fiber singlefilament properties which ultimately produces a composite articleexhibiting a diminished tensile strength.

More recently, various liquid oxidative surface treatments for carbonfibers have been proposed. Illustrative examples of representativenonelectrolytic treatments utilizing an aqueous sodium hypochloritesolution are disclosed in British Pat. No. 1,238,308, and German Pat.Nos. 2,112,455 and 2,147,419. Illustrative examples of electrolyticliquid oxidative surface treatments are disclosed in British Pat. No.1,257,022, Belgian Patent No. 747,631, and German Pat. No. 2,048,916.The nonelectrolytic treatments identified above commonly employ elevatedprocessing temperatures, extended treatment times, and relativelyunstable solutions with which the carbon fibers are treated. Theelectrolytic treatments identified above commonly employ highly elevatedcurrent densities and relatively unstable solutions with which thecarbon fibers are treated. It has been recognized by those skilled inthe art that high modulus graphitic carbonaceous fibers have proven tobe more difficult to surface treat than low modulus carbon fibers.

It is an object of the invention to provide an improved electrolyticprocess for efficiently modifying the surface characteristics of highmodulus graphitic carbonaceous fibers.

It is an object of the invention to provide an improved electrolyticprocess for enhancing the ability of high modulus graphitic carbonaceousfibers to bond to a resinous matrix material.

It is an object of the invention to provide an improved electrolyticprocess for modifying the surface characteristics of high modulusgraphitic carbonaceous fibers which may be conducted relatively rapidlyat relatively low current densities.

It is an object of the invention to provide an improved electrolyticliquid phase oxidation process for modifying the surface characteristicsof high modulus graphitic carbonaceous fibers in the absence of anysubstantial diminution in the tensile properties thereof.

It is an object of the invention to provide an improved electrolyticliquid phase oxidation process for modifying the surface characteristicsof high modulus graphitic carbonaceous fibers wherein the fibers aretreated with an aqueous solution of sodium hypochlorite. i

It is an object of the invention to provide composite articlesexhibiting an improved interlaminar shear strength reinforced with highmodulus carbon fibers.

These and other objects, as well as the scope, nature, and utilizationof the invention will be apparent from the following detaileddescription and appended claims.

SUMMARY OF THE INVENTION It has been found that an improved electrolyticprocess for enhancing the ability of an electrically conductivecarbonaceous fibrous material containing at least about percent carbonby weight and exhibiting a mean single filament Youngs modulus of atleast about 60 million psi and a predominantly graphitic x-raydiffraction pattern to bond to a resinous matrix material comprises:

a. immersing the fibrous material in an aqueous electrolytic solution ofsodium hypochlorite having a pH of about 8 to 12 and an active chlorineconcentration of about 1 to 7 percent by weight,

b. providing a cathode in contact with the aqueous electrolytic solutionand in a spaced relationship to the immersed fibrous material,

c. applying electrical current to the fibrous material while immersed inthe aqueous electrolytic solution of sodium hypochlorite at a currentdensity of about 2.5 to 12 milliamps per square centimeter of surfacearea of the immersed fibrous material for a residence time of about 2 tominutes with the fibrous material serving as an anode,

d. washing the resulting carbonaceous fibrous material to removeresidual aqueous electrolytic solution adhering to the same, and

e. drying the same.

The resulting high modulus graphitic carbonaceous fibers may beincorporated in a matrix material to form a composite article exhibitingan enhanced interlaminar shear strength.

DESCRIPTION OF DRAWING The drawing is a schematic presentation of anapparatus arrangement capable of carrying out the improved process ofthe present invention on a continuous basis.

DESCRIPTION OF PREFERRED EMBODIMENTS The Starting Material The graphiticcarbonaceous fibrous materials which are modified in accordance with thepresent process contain at least about 90 percent carbon by weight andexhibit a predominantly graphitic carbon x-ray diffraction pattern. In apreferred embodiment of the process the graphitic carbonaceous fibrousmaterials which undergo surface treatment contain at least about 95percent carbon by weight, and at least about 99 percent carbon byweightin a particularly preferred embodiment of the process.

The fibers which are modified in accordance with the present processadditionally exhibit a relatively high mean single filament Youngsmodulus of at least about 60 million psi, e.g., about 60 to 90 millionpsi, and preferably a mean single filament Youngs modulus of about 70 to90 million psi. Additionally, the fibers commonly exhibit a singlefilament tensile strength of at least about 250,000 psi, e.g., about300,000 to 400,000 psi. The Youngs modulus of the fiber may bedetermined, for instance, by the procedure of ASTM DesignationD-2lOl-64T.

The graphitic carbonaceous fibrous materials may be present as acontinuous length and may be provided in a variety of physicalconfigurations so long as substantial access to the fiber surface ispossible during the surface modification treatment described hereafter.For instance, the graphitic carbonaceous fibrous materials may assumethe configuration of a continuous length of a multifilament yarn, tape,tow, strand, cable, or similar fibrous assemblage. In a preferredembodiment of the process the graphitic carbonaceous fibrous material isone or more continuous multifilament yarn or tow.

The graphitic carbonaceous fibrous material which is treated in thepresent process optionally may be provided with a twist of about 0.l to5tpi (turns per inch), and preferably about 0.3 to 1.0 tpi, may beimparted to a multifilament yarn. Also, a false twist may be usedinstead of or in addition to a real twist. Alternatively, one

may select continuous bundles of fibrous material which possessessentially no twist.

The graphitic carbonaceous fibers which serve as the starting materialin the present process may be formed in accordance with a variety oftechniques as will be apparent to those skilled in the art. Forinstance, organic polymeric fibrous materials which are capable ofundergoing thermal stabilization may be initially stabilized bytreatment in an appropriate atmosphere at a moderate temperature (e.g.,200 to 400C), and subsequently heated in an inert atmosphere to a morehighly elevated temperature, e.g., l,500 to 2,000C., or more, until agraphitic carbonaceous fibrous material is formed. The higher thetemperature the greater the amount of graphitic carbon produced withinthe same.

The exact temperature and atmosphere utilized during the initialstabilization of an organic polymeric fibrous material commonly varywith the composition of the precursor as will be apparent to thoseskilled in the art. During the subsequent carbonization reactionelements present in the fibrous material other than carbon (e.g., oxygenand hydrogen) are substantially expelled. Suitable organic polymericfibrous materials from which the fibrous material capable of undergoingcarbonization and graphitization may be derived include an acrylicpolymer, a cellulosic polymer, a polyamide, a polybenzimidazole,polyvinyl alcohol, etc. As discussed hereafter, acrylic polymericmaterials are particularly suited for use as precursors in the formationof graphitic carbonaceous fibrous materials. Illustrative examples ofsuitable cellulosic materials include the natural and regenerated formsof cellulose, e.g., rayon. Illustrative examples of suitable polyamidematerials include the aromatic polyamides, such as nylon 6T, which isformed by the condensation of hexamethylenediamine and terephthalicacid. An illustrative example of a suitable polybenzimidazole ispoly-2,2'-mphenylene-S,S-bibenzimidazole.

A fibrous acrylic polymeric material prior to stabilization may beformed primarily of recurring acrylonitrile units. For instance, theacrylic polymer should contain not less than about mole percent ofrecurring acrylonitrile units with not more than about 15 mole percentof a monovinyl compound which is copolymerizable with acrylonitrile suchas styrene, methyl acrylate, methyl methacrylate, vinyl acetate, vinylchloride, vinylidene chloride, vinyl pyridine, and the like, or aplurality of such monovinyl compounds.

During the formation of a preferred carbonaceous fibrous material foruse in the present process multifilament bundles of an acrylic fibrousmaterial may be ini tially stabilized in an oxygen-containing atmosphere(i.e., preoxidized) on a continuous basis in accordance with theteachings of US. Ser. No. 749,957, filed Aug. 5, 1968, of Dagobert E.Stuetz, which is assigned to the same assignee as the present inventionand is herein incorporated by reference. More specifically, the acrylicfibrous material should be either an acrylonitrile homopolymer or anacrylonitrile copolymer which contains no more than about 5 mole percentof one or more monovinyl comonomers copolymerized with acrylonitrile. Ina particularly preferred embodiment of the process the fibrous materialis derived from an acrylonitrile homopolymer. The stabilized acrylicfibrous material which is preoxidized in an oxygen-containing atmosphereis black in appearance, contains a bound oxygen content of at leastabout 7 percent by weight as determined by the Unterzaucher analysis,retains its original fibrous configuration essentially intact, and isnonburning when subjected to an ordinary match flame. Another preferredstabilization technique is disclosed in commonly assigned US. Pat. No.3,508,874 of Richard N. Rulison. A preferred carbonization andgraphitization technique is disclosed in commonly asssigned U.S. SerialNo. 244,990, filed Apr. 17, 1972 of Charles M. Clarke which is hereinincorporated by reference.

In accordance with a particularly preferred carbonization andgraphitization technique a continuous length of stabilized acrylicfibrous material which is non-burning when subjected to an ordinarymatch flame and derived from an acrylic fibrous material selected fromthe group consisting of an acrylonitrile homopolymer and acrylonitrilecopolymers which contain at least about 85 mole percent of acrylonitrileunits and up to about mole percent of one or more monovinyl unitscopolymerized therewith is converted to a graphitic fibrous materialwhile preserving the original fibrous configuration essentially intactwhile passing through a carbonization/graphitization heating zonecontaining an inert gaseous atmosphere and a temperature gradient inwhich the fibrous material is raised within a period of about to about300 seconds from about 800C. to a temperature of about 1,600C. to form acontinuous length of carbonized fibrous material, and in which thecarbonized fibrous material is subsequently raised from about 1,600C. toa maximum temperature of at least about 2,400C. within a period of about3 to 300 seconds where it is maintained for about 10 seconds to about200 seconds to form a continuous length of graphitic fibrous material.

The equipment utilized to produce the heating zone used to produce thegraphitic carbonaceous starting material may be varied as will beapparent to those skilled in the art. It is essential that the apparatusselected be capable of producing the required temperature whileexcluding the presence of an oxidizing atmosphere.

ln a preferred technique the continuous length of fibrous materialundergoing carbonization is heated by use of an induction furnace. Insuch a procedure the fibrous material may be passed in the direction ofits length through a hollow graphite tube or other susceptor which issituated within the windings of an induction coil. By varying the lengthof the graphite tube, the length of the induction coil, and the rate atwhich the fibrous material is passed through the graphite tube, manyapparatus arrangements capable of producing carbonization andgraphitization may be selected. For large scale production, it is ofcourse preferred that relatively long tubes or susceptors be used sothat the fibrous material may be passed through the same at a more rapidrate while being carbonized and graphitized. The temperature gradient ofa given apparatus may be determined by conventional optical pyrometermeasurements as will be apparent to those skilled in the art. Thefibrous material because of its small mass and relatively large surfacearea instantaneously assumes essentially the same temperature as that ofthe zone through which it is continuously passed.

The Surface Treatment The graphitic carbonaceous fibrous material(heretofore described) is subjected to a relatively brief electrolytictreatment while immersed in an aqueous solution of sodium hypochloriteas described in detail hereafter, and the resulting fibrous material iswashed and dried. During the electrolytic treatment the graphiticcarbonaceous fibrous material serves as an anode. A cathode is providedin contact with the aqueous solution of sodium hypochlorite and in aspaced relationship to the immersed graphitic carbonaceous fibrousmaterial.

The aqueous solution of sodium hypochlorite in which the graphiticcarbonaceous fibrous material is immersed during the electrolytictreatment has a pH of about 8 to 12, preferably 10.5 to 11.5 (e.g.,about ll), and an active chlorine concentration (i.e., an availablechlorine concentration) of about 1 to 7 percent by weight, preferablyabout 3 to 7 percent by weight, and most preferably about 5.025 to 5.4percent by weight (e.g., 5.25 percent by weight). The active chlorineconcentration for a given solution of sodium hypochlorite may bedetermined by titration with sodium thiosulfate after addition of excessKl. Commercially available liquid bleach meeting the above prerequisitesmay be selected for use in the present process, and is sometimesdesignated as soda bleach liquor or simply as household bleach solution.Such a solution may be formed, inter alia, by passage of chlorinethrough a dilute caustic soda solution in either a batch or continuousoperation in accordance with techniques known in the art. The sodiumhypochlorite solution utilized in the process of the present inventionis considerably more stable than common laundry grade commercial bleachsolutions which contain 12 to 15 percent active chlorine. For optimumstability a sodium hypochlorite solution having a pH of about i l isselected. As the electrolytic treatment progresses, active chlorine isconsumed and the pH of the solution decreases..The active chlorineconcentration of the solution may accordingly be replenished eithercontinuously or intermittantly so that the active chlorine concentrationand the pH are substantially maintained at the desired level throughoutthe electrolytic treatment. For instance, the active chlorineconcentration may be conveniently replenished by feeding fresh aqueouselectrolytic solution and withdrawing a portion of the spent solution.The active chlorine concentration should not fall below about 1 percentby weight, preferably should not fall below about 3 percent by weight,and most preferably should not fall below about 5.025 percent by weightduring any substantial portion of the electrolytic treatment. The pH ofthe solution preferably should not fall below about 8, and mostpreferably should not fall below about during any substantial portion ofthe electrolytic treatment.

During the process of the present invention an electrical current isapplied to the graphitic carbonaceous fibrous material while immersed inthe aqueous electrolytic solution of sodium hypochlorite at a relativelylow current density of about 2.5 to 12 milliamps per square centimeterof surface area of said immersed fibrous material, and preferably acurrent density of about 4 to 12 milliamps per square centimeter ofsurface area of said immersed fibrous material. At current densitiesmuch below about 2.5 milliamps the desired surface treatment has beenfound to be inordinately slow. At current densities much above about 12milliamps the rate of the desired surface modification is notappreciably increased and a significant fire hazard results because ofthe formation of hydrogen at the cathode via overvoltage. The surfacearea of the graphitic carbonaceous fibrous material undergoing treatmentmay be determined by the aid of BET analysis, or any other conventionaltechnique. For instance, the specific surface area of graphiticcarbonaceous fibrous material may be determined by BET analysis andmultiplied by the number of grams of fibrous material immersed in theaqueous electrolytic solution.

The solution preferably is provided at the mild temperature of about 20to 35C. when contacted with the graphitic carbonaceous fibrous material,and most preferably is provided at room temperature (i.e., at about25C.). More highly elevated temperatures have been found to lead toprocess instability.

The improved electrolytic process of the present invention surprisinglyproduces the desired surface modification of the graphitic carbonaceousfibrous material in the brief residence time of about 2 to 10 minutes inspite of the relatively mild current density utilized. The exactresidence time for optimum results will vary somewhat with the Youngsmodulus of the graphitic carbonaceous fibrous material undergoingtreatment. Generally, the higher the mean Youngs modulus above about 60million psi the longer the residence time employed for optimum results.Commonly residence times of about 2 to 6 minutes are utilized with agraphitic carbonaceous fibrous material having a mean Youngs modulus of60 to 90 million psi. For instance, a resi dence time of about 2 minutesmay be selected while operating at a current density of about 12milliamps per square centimeter of surface area of said immersed fibrousmaterial, and a residence time of about 5 to 6 minutes may be selectedwhile operating at a current density of about 4 milliamps per squarecentimeter of surface area of said immersed fibrous material.

During the electrolytic treatment in the aqueous solution of sodiumhypochlorite adequate solution is provided so that the fibrous materialis completely immersed inthe same.

The electrolytic surface modification process of the present inventionmay be carried out on either a batch or a continuous basis. Forinstance, an electrical contact may be secured to the graphiticcarbonaceous fibrous material and the fibrous material immersed in asuitable aqueous solution of sodium hypochlorite in which a cathode isalso provided. Preferably the process is carried out on a continuousbasis with a continuous length of the fibrous material beingcontinuously passed through the aqueous solution of sodium hypochlorite.in such an embodiment the continuous length of fibrous material may passover one or more electrical contact (e.g., idler or driven rollers)through which a current of an appropriate current density is supplied.

Following the surface modification treatment heretofore described theresulting graphitic carbonaceous fibrous material is washed so as toremove residual quantities of the sodium hypochlorite solution adheringto the same. The washing may be carried out in any convenient manner andshould be as exhaustive as possible since residual sodium hypochloriteif left on the fiber will adversely influence the properties of acomposite incorporating the same. In a preferred wash technique thewashing is conducted by initially contacting the resulting graphiticcarbonaceous fibrous material with a solution of a dilute acid, and bysubsequently rinsing the same with water. The acid serves to neutralizeany adhering residue and to aid in its expeditious removal. Forinstance, dilute mineral acids such as hydrochloric acid, sulfuric acid,etc. may be employed. Also, acetic acid may be conveniently selected.The washing may be carried out on a static or a continuous basis whereina continuous length of the fibrous material is passed through one ormore wash solutions.

Following washing and prior to utilization as fibrous reinforcement in acomposite article, the surface modified carbonaceous fibrous material isdried to remove any adhering wash solution. Such drying may be simplyconducted by placing the same in a circulating air oven provided atabout to 250C.

The theory whereby the high modulus carbonaceous fibrous materialheretofore defined may be beneficially surface modified under therelatively mild electrolytic surface treatment conditions and briefresidence times disclosed herein is considered complex and incapable ofsimple explanation. Additionally, the ability of one to produce thedesired surface modification utilizing the conditions heretofore recitedis considered to be most surprising when compared with the considerablymore severe electrolytic surface modification conditions employed byothers in the prior art.

The surface modification imparted to the graphitic carbonaceous fibrousmaterial through the use of the present process has been found toexhibit an appreciable life which is not diminished to any substantialdegree even after the passage of 30, or more days. Also, the singlefilament tensile properties of the carbonaceous fibrous material are notadversely influenced by the surface modification treatment of thepresent invention, and the surface of the resulting fibrous material issubstantially free of pitting.

The surface treatment of the present process makes possible improvedadhesive bonding between the graphitic carbonaceous fibers, and aresinous matrix material. Accordingly, carbon fiber reinforced compositematerials which incorporate fibers treated as heretofore describedexhibit an enhanced interlaminar shear strength, fiexural strength,compressive strength, etc. The resinous matrix material employed in theformation of such composite materials is commonly a polar thermosettngresin such as an epoxy, a polyimide, a polyester, a phenolic, etc., or athermoplastic resin. The graphitic carbonaceous fibrous material iscommonly provided in such resulting composite materials in either analigned or random fashion in a concentration of about 20 to 70 percentby volume.

The following examples are given as specific illustrations of theinvention. it should be understood, however, that the invention is notlimited to the specific details set forth in the examples.

EXAMPLE I A high strength-high modulus continuous filament graphiticcarbonaceous yarn derived from an acrylonitrile homopolymer was selectedas the starting material for use in the present process. The graphiticcarbonaceous yarn was provided as a continuous length and consisted of25 ends twisted in a yarn bundle at a rate of 0.5 turn per inch. Eachend consisted of about 385 continuous filaments, each having a denierper filament of about 0.9'. The graphitic carbonaceous yarn was formedby heating the acrylonitrile homopolymer fibers in air at a temperatureof about 270C. for about minutes, and by subsequently heating theresulting stabilized yarn in a circulating nitrogen atmosphere to amaximum temperature of about 2,650C. in accordance with the teachings ofcommonly assigned U.S. Ser. No. 244,990, filed Apr. 17, 1972, of CharlesM. Clarke which is herein incorporated by reference.

The yarn prior to surface treatment in accordance with the presentprocess exhibited a carbon content of about 99 percent by weight, a meansingle filament Youngs modulus of about 76 million psi, a density of1.925 grams/cc, and a mean single filament tenacity of about 318,000psi. The yarn exhibited a predominantly graphitic x-ray diffractionpattern.

The yarn l was supplied by feed bobbin 2 and continuously was passed inthe directon of its length through the apparatus illustrated in thedrawing wherein it was immersed in an aqueous solution of sodiumhypochlorite 4. The yarn was wrapped about grooved graphite roller 8 andgrooved polyvinylchloride roller 10 as it passed through the apparatus.The rollers 8 and 10 were mounted on supports 12 and 14 provided in apolymethylmethacrylate vessel 6. The aqueous solution of sodiumhypochlorite 4 was provided at room temperature (i.e., at about 25C.),had a pH of 11, and an active chlorine concentration of 5.25 percent byweight. The aqueous solution of sodium hypochlorite was obtained underthe designation of Clorox household bleach solution. While immersed inaqueous solution for a residence time of about 9 minutes, a DC currentwas applied to the yarn via contact with graphite roller 8 at a currentdensity of 2.7 milliamps (i.e., 0.0027 amps) per square centimeter ofsurface area of the immersed yarn. The graphite roller 8 was connectedto power source 16 by electrical lead 18. The cathode consisted of apair of graphite plates 20 and 22 which were also immersed in the sodiumhypochlorite solution 4 and were connected to the power source 16 byelectrical leads 24 and 26. The resulting surface treated yarn 28 wastaken up on rotating bobbin 30. At the conclusion of the surfacetreatment the aqueous solution exhibited a pH of 9.6 and an activechlorine concentration of 4.6 percent by weight.

Following the immersion in the aqueous solution of sodium hypochlorite,the yarn was washed by (a) immersing the same in flowing tap water forminutes, (b) contacting the same with a 2 percent hydrochloric acidsolution provided in distilled water for about 1 minute, and (c) rinsingthe same with deionized water for 15 minutes. The yarn was next dried ina forced air circulating oven at 70C. for approximately 16 hours. Thetensile properties of the resulting fibers were substantially unchanged.More specifically, the mean single filament Youngs modulus of thesurface treated yarn was 75 million psi, and the mean single filamenttenacity was unchanged.

A composite article was next formed employing the surface modified yarnas a reinforcing media in an epoxy resin matrix. The composite articlewas a rectangular bar consisting of about 59.7 percent by volume of theyarn and having dimensions of /a inch X )4 inch X 5 inches. Thecomposite article was formed by impregnation of the yarn in a liquidepoxy resinhardner mixture at 80C., followed by unidirectional layup ofthe required quantity of the impregnated yarn in a steel mold, andcompression molding of the layup for 2 hours at 93C., and 2.5 hours at200C. in a heated platen press. The mold was cooled slowly to roomtemperature, and the composite article was removed from the mold cavityand cut to size for testing. The resinous matrix material used in theformation of the composite article was provided as a solventless systemwhich contained 100 parts by weight of epoxy resin and about 87 parts byweight of anhydride curing agent.

The resulting composite article exhibited a horizontal interlaminarshear strength of 8,540 psi, a flexural strength of 128,000 psi, and aflexural modulus of 34,000,000 psi.

The horizontal interlaminar shear strength was determined by short beamtesting of the fiber reinforced composite according to the procedure ofASTM D2344-65T as modified for straight bar testing at a 4 l span todepth ratio. The flexural strength of the fiber reinforced composite wasdetermined by four point bending according to the procedure of ASTMD2344 at a 3.2 l span to depth ratio. The modulus was also determined byfour point bending.

For comparative purposes Example 1 was repeated with the exception thatthe high strength-high modulus continuous filament grpahiticcarbonaceous yarn underwent no form of surface modification prior toincorporation in the composite article. The resulting composite articleexhibited a horizontal interlaminar shear strength of only 3150 psi, aflexural strength of 121,000 psi, and a flexural modulus of 35,200,000psi.

EXAMPLE ll Example I was repeated with the exception that the highstrengthhigh modulus continuous filament graphitic carbonaceous yarn wassurface modified while immersed in the aqueous solution of sodiumhypochlorite for a time of about 5.5 minutes at a current density of 3.6milliamps (i.e., 0.0036 amps) per square centimeter of surface area ofthe immersed yarn. At the conclusion of the surface treatment theaqueous solution exhibited a pH of 9.44 and an active chlorineconcentration of 4.19percent by weight. The resulting composite articleexhibited a horizontal interlaminar shear strength of 9340 psi, aflexural strength of 121,000 psi, and a flexural modulus of 37,500,000psi.

EXAMPLE Ill Example I was repeated with the exception that the highstrengthhigh modulus continuous filament graphitic carbonaceous yarn wassurface modified while immersed in the aqueous solution of sodiumhypochlorite for a residence time of 5.5 minutes at a current density ofabout 4 milliamps (i.e., about 0.004 amps) per square centimeter ofsurface area of the immersed yarn. At the conclusion of the surfacetreatment the aqueous solution exhibited a pH of 9.44 and an activechlorine concentration of 4.12 percent by weight. The resulting articleexhibited a horizontal interlaminar shear strength of 10,500 psi, aflexural strength of 1 17,000 psi, and a flexural modulus of 34,000,000psi.

Although the invention has been described with preferred embodiments, itis to be understood that variations and modifications may be resorted toas will be apparent to those skilled in the art. Such variations are tobe considered within the purview and scope of the claims appendedhereto.

We claim:

1. An improved electrolytic process for enhancing the ability of anelectrically conductive carbonaceous fibrous material containing atleast about 90 percent carbon by weight and exhibiting a mean singlefilament Youngs modulus of at least about 60 million psi and apredominantly graphitic x-ray diffraction pattern to bond to a resinousmatrix material comprising:

a. immersing said fibrous material in an aqueous electrolytic solutionof sodium hypochlorite having a pH of about 8 to 12 and an activechlorine concentration of about 1 to 7 percent by weight,

b. providing a cathode in contact with said aqueous electrolyticsolution and in a spaced relationship to I said immersed fibrousmaterial,

c. applying electrical current to said fibrous material while immersedin said aqueous electrolytic solution of sodium hypochlorite at acurrent density of about 2.5 to 12 milliamps per square centimeter ofsurface area of said immersed fibrous material for a residence time ofabout 2 to minutes with said fibrous material serving as an anode,

d. washing the resulting carbonaceous fibrous material to removeresidual aqueous electrolytic solution adhering to the same, and

e. drying the same.

2. An improved process for enhancing the ability of a carbonaceousfibrous material to bond to a resinous matrix material in accordancewith claim 1 wherein said carbonaceous fibrous material contains atleast about 95 percent carbon by weight.

3. An improved process for enhancing the ability of a carbonaceousfibrous material to bond to a resinous matrix material in accordancewith claim 1 wherein said carbonaceous fibrous material exhibits a meansingle filament Youngs modulus of about 60 to 90 million 4. An improvedprocess for enhancing the ability of a carbonaceous fibrous material tobond to a resinous matrix material in accordance with claim 1 whereinsaid carbonaceous fibrous material is continuously passed through saidaqueous electrolytic solution in the direction of its length.

5. An improved process for enhancing the ability of a carbonaceousfibrous material to bond to a resinous matrix material in accordancewith claim 1 wherein said aqueous electrolytic solution of sodiumhypochlorite has an active chlorine concentration of about 3 to 7percent by weight.

6. An improved process for enhancing the ability of a carbonaceousfibrous material to bond to a resinous matrix material in accordancewith claim 1 wherein said aqueous solution of sodium hypochlorite has apH of about 10.5 to 1 1.5 and an active chlorine concentration of about5.025 to 5.4 percent by weight.

7. An improved process for enhancing the ability of a carbonaceousfibrous material to bond to a resinous matrix material according toclaim 1 wherein said aqueous electrolytic solution is provided at atemperature of about to 35C.

8. An improved process for enhancing the ability of a carbonaceousfibrous material to bond to a resinous matrix material according toclaim 1 wherein said electrical current is applied to said fibrousmaterial for about 2 to 6 minutes.

9. An improved process for enhancing the ability of a carbonaceousfibrous material to bond to a resinous matrix according to claim 1wherein said washing is conducted by initially contacting said resultingcarbonaceous fibrous material with a solution of a dilute acid, and bysubsequently rinsing the same with water.

10. An improved process for enhancing the ability of a carbonaceousfibrous material containing at least about 95 percent carbon by weightand exhibiting a mean single filament Youngs modulus of about 60 to 90million psi and a predominantly graphitic x-ray diffraction pattern tobond to a resinous matrix material comprising:

a. immersing said fibrous material in an aqueous electrolytic solutionof sodium hypochlorite provided at about 20 to 35C. having a pH of about10.5 to 11.5 and an active chlorine concentration of about 5.025 to 5.4percent by weight,

b. providing a cathode in contact with said aqueous electrolyticsolution and in a spaced relationship to said immersed fibrous material,

c. applying electrical current to said fibrous material while immersedin said aqueous electrolytic solution of sodium hypochlorite at acurrent density of about 4 to 12 milliamps per square centimeter ofsurface area of said immersed fibrous material for a residence time ofabout 2 to 6 minutes with said fibrous material serving as an anode,

d. washing the resulting carbonaceous fibrous material to removeresidual aqueous electrolytic solution adhering to the same, and

e. drying the same.

11. An improved process for enhancing the ability of a carbonaceousfibrous material to bond to a resinous matrix material in accordancewith claim 10 wherein said carbonaceous fibrous material is continuouslypassed through said aqueous electrolytic solution in the direction ofits length.

12. An improved process for enhancing the ability of a carbonaceousfibrous material to bond to a resinous matrix material in accordancewith claim 10 wherein said electrolytic aqueous solution of sodiumhypochlorite is provided at about 25C.

13. An improved process for enhancing the ability of a carbonaceousfibrous material to bond to a resinous matrix material in accordancewith claim 10 wherein said washing is conducted by initially contactingsaid resulting carbonaceous fibrous material with a solution of a diluteacid, and by sebsequently rinsing the same with water.

14. An improved process for enhancing the ability of a carbonaceousfibrous material containing at least about 95 percent carbon by weightand exhibiting a mean single filament Youngs modulus of about to millionpsi and a predominantly graphitic x-ray diffraction pattern to bond to aresinous matrix material comprising:

a. immersing said fibrous material in an aqueous solution of sodiumhypochlorite provided at about 25C. having a pH of about 1 1 and anactive chlorine concentration of about 5.025 to 5.4 percent by weight,

b. providing a cathode in contact with said aqueous electrolyticsolution and in a spaced relationship to said immersed fibrous material,

c. applying electrical current to said fibrous material while immersedin said aqueous electrolytic solution of sodium hypochlorite at acurrent density of about 4 milliamps per square centimeter of surfacearea of said immersed fibrous material for a residence time of about 5to 6 minutes with said fibrous material serving as an anode,

d. washing the resulting carbonaceous fibrous material to removeresidual aqueous electrolytic solution adhering to the same, and

16. A composite article exhibiting enhanced interlaminar shear strengthcomprising a resinous matrix material having incorporated therein acarbonaceous fibrous material having its surface characteristicsmodified in accordance with the process of claim 1.

1. AN IMPROVED ELECTROLYTIC PROCESS FOR ENHANCING THE ABILITY OF ANELECTRICALLY CONDUCTIVE CARBONACEOUS FIBROUS MATERIAL CONTAINING ATLEAST ABOUT 90 PERCENT CARBON BY WEIGHT AND EXHIBITING A MEAN SINGLEFILAMENT YOUNG''S MODULUS OF AT LEAST ABOUT 60 MILLION PSI AND APREDOMINANTLY GRAPHITIC X-RAY DIFFRACTION PATTERN TO BOND TO A RESINOUSMATRIX MATERIAL COMPRISING: A. IMMERSING SAID FIBROUS MATERIAL IN ANAQUEOUS ELECTROLYTIC SOLUTION OF SODIUM HYPOCHLORITE HAVING A PH OFABOUT 8 TO 12 AND AN ACTIVE CHLORINE CONCENTRATION OF ABOUT 1 TO 7PERCENT BY WEIGHT, B. PROVIDING A CATHODE IN CONTACT WITH SAID AQUEOUSELECTROLYTIC SOLUTION AND IN A SPACED RELATIONSHIP TO SAID IMMERSEDFIBROUS MATERIAL,
 2. An improved process for enhancing the ability of acarbonaceous fibrous material to bond to a resinous matrix material inaccordance with claim 1 wherein said carbonaceous fibrous materialcontains at least about 95 percent carbon by weight.
 3. An improvedprocess for enhancing the ability of a carbonaceous fibrous material tobond to a resinous matrix material in accordance with claim 1 whereinsaid carbonaceous fibrous material exhibits a mean single filamentYoung''s modulus of about 60 to 90 million psi.
 4. An improved processfor enhancing the ability of a carbonaceous fibrous material to bond toa resinous matrix material in accordance with claim 1 wherein saidcarbonaceous fibrous material is continuously passed through saidaqueous electrolytic solution in the direction of its length.
 5. Animproved process for enhancing the ability of a carbonaceous fibrousmaterial to bond to a resinous matrix material in accordance with claim1 wherein said aqueous electrolytic solution of sodium hypochlorite hasan active chlorine concentration of about 3 to 7 percent by weight. 6.An improved process for enhancing the ability of a carbonaceous fibrousmaterial to bond to a resinous matrix material in accordance with claim1 wherein said aqueous solution of sodium hypochlorite has a pH of about10.5 to 11.5 and an active chlorine concentration of about 5.025 to 5.4percent by weight.
 7. An improved process for enhancing the ability of acarbonaceous fibrous material to bond to a resinous matrix materialaccording to claim 1 wherein said aqueous electrolytic solution isprovided at a temperature of about 20* to 35*C.
 8. An improved processfor enhancing the ability of a carbonaceous fibrous material to bond toa resinous matrix material according to claim 1 wherein said electricalcurrent is applied to said fibrous material for about 2 to 6 minutes. 9.An improved process for enhancing the ability of a carbonaceous fibrousmaterial to bond to a resinous matrix according to claim 1 wherein saidwashing is conducted by initially contacting said resulting carbonaceousfibrous material with a solution of a dilute acid, and by suBsequentlyrinsing the same with water.
 10. An improved process for enhancing theability of a carbonaceous fibrous material containing at least about 95percent carbon by weight and exhibiting a mean single filament Young''smodulus of about 60 to 90 million psi and a predominantly graphiticx-ray diffraction pattern to bond to a resinous matrix materialcomprising: a. immersing said fibrous material in an aqueouselectrolytic solution of sodium hypochlorite provided at about 20* to35*C. having a pH of about 10.5 to 11.5 and an active chlorineconcentration of about 5.025 to 5.4 percent by weight, b. providing acathode in contact with said aqueous electrolytic solution and in aspaced relationship to said immersed fibrous material, c. applyingelectrical current to said fibrous material while immersed in saidaqueous electrolytic solution of sodium hypochlorite at a currentdensity of about 4 to 12 milliamps per square centimeter of surface areaof said immersed fibrous material for a residence time of about 2 to 6minutes with said fibrous material serving as an anode, d. washing theresulting carbonaceous fibrous material to remove residual aqueouselectrolytic solution adhering to the same, and e. drying the same. 11.An improved process for enhancing the ability of a carbonaceous fibrousmaterial to bond to a resinous matrix material in accordance with claim10 wherein said carbonaceous fibrous material is continuously passedthrough said aqueous electrolytic solution in the direction of itslength.
 12. An improved process for enhancing the ability of acarbonaceous fibrous material to bond to a resinous matrix material inaccordance with claim 10 wherein said electrolytic aqueous solution ofsodium hypochlorite is provided at about 25*C.
 13. An improved processfor enhancing the ability of a carbonaceous fibrous material to bond toa resinous matrix material in accordance with claim 10 wherein saidwashing is conducted by initially contacting said resulting carbonaceousfibrous material with a solution of a dilute acid, and by sebsequentlyrinsing the same with water.
 14. An improved process for enhancing theability of a carbonaceous fibrous material containing at least about 95percent carbon by weight and exhibiting a mean single filament Young''smodulus of about 70 to 90 million psi and a predominantly graphiticx-ray diffraction pattern to bond to a resinous matrix materialcomprising: a. immersing said fibrous material in an aqueous solution ofsodium hypochlorite provided at about 25*C. having a pH of about 11 andan active chlorine concentration of about 5.025 to 5.4 percent byweight, b. providing a cathode in contact with said aqueous electrolyticsolution and in a spaced relationship to said immersed fibrous material,c. applying electrical current to said fibrous material while immersedin said aqueous electrolytic solution of sodium hypochlorite at acurrent density of about 4 milliamps per square centimeter of surfacearea of said immersed fibrous material for a residence time of about 5to 6 minutes with said fibrous material serving as an anode, d. washingthe resulting carbonaceous fibrous material to remove residual aqueouselectrolytic solution adhering to the same, and e. drying the same. 15.An improved process for enhancing the ability of a carbonaceous fibrousmaterial to bond to a resinous matrix material in accordance with claim14 wherein said carbonaceous fibrous material is continuously passedthrough said aqueous electrolytic solution in the direction of itslength.
 16. A composite article exhibiting enhanced interlaminar shearstrength comprising a resinous matrix material having incorporatedtherein a carbonaceous fibrous material having its surfacecharacteristics modified in accordance wiTh the process of claim 1.